Safe storage of petroleum products and drinking water in areas subject to earthquake has long been a subject of intensive study and experimentation. The most common devices for safeguarding the storage tanks are hydraulic dampers which are located between points of the supporting framework that are subject to relative movement. Such hydraulic devices are exceedingly complex and expensive, require continuous maintenance, and have limited effectiveness.
U.S. Pat. No. 3,605,953, of which the present inventor is one of four inventors, discloses a damping unit for a building or structure which is subject to subsonic oscillations such as may be produced by the wind or by earth movements. That damping unit comprises at least two rigid members and a layer of viscoelastic material bonded between broad surfaces of each pair of rigid members. By attaching such a damping unit between two points of the structure which experience relative motion when it oscillates due to wind or earth movement, the oscillation produces shear displacement in the viscoelastic layer which absorbs energy by virtue of its "lossy" nature. For effective damping, the viscoelastic material should have a loss tangent of at least 0.5 measured at the operating temperature of the damping unit and at the frequency of oscillation to be damped. A high building may oscillate in the wind at a frequency approximately 0.1 cycle per second whereas the frequency of an earthquake may be from 0.5 to 10 cycles per second.